Wheel alignment of vehicles encompasses measurement, calculation and determination of several different angles related to the wheels and wheel axles of the vehicles. Examples of such angles that are of importance for maintaining or enhancing security, functionality and economics of vehicles are; toe, steering wheel centering, camber, caster, king-pin inclination, toe-out on turn and max turn.
In addition and especially for heavier vehicles such as for lorries, trucks, busses, towcars, trailers and semitrailers, the out of square of wheel axles, especially of rear wheel axles, is of great importance. Out of square, sometimes referred to as thrust angle, is defined as an angle formed between the longitudinal centreline of the vehicle and the so called trust line of the wheel axle in question. The trust line in turn is defined as the normal to the longitudinal axis of the wheel axle. In practice however the out of square for a wheel axle is normally defined and calculated as the difference between the toe angle for the right and left wheel divided by two. Irrespective of how the out of square is defined and calculated it is of great importance that the longitudinal geometrical centreline of the vehicle can be accurately determined for allowing accurate determining of the out of square.
It is further of great importance to be able to determine and, by adjustments made to the vehicle, minimize the out of square, since any out of square differing from zero substantially increases tyre wear and fuel consumption of the vehicle. Out of square other that zero also increases the effective widths occupied by the vehicle when travelling on a road. This in turn increases the air resistance of the vehicle which results in further increased fuel consumption and deteriorates road safety.